Oxidative Phosphorylation and Role of the Electron Transport Chain

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Describe the importance of mitochondria in the cell

Mitochondria are the major site for aerobic respiration (requires molecular O2) and the production of energy They are in higher concentrations in cells with high energy demands like liver and muscle.

Describe the structural features of mitochondria

Mitochondria have an outer membrane which is relatively permeable to small molecules and ions due to porin.

• They have an inner membrane which is impermeable to most small molecules and ions and has multiple folds called cristae to increase surface area.

• The matrix is the innermost compartment containing major metabolic enzymes, including those for the TCA cycle and fatty acid oxidation, as well as genetic material.

• There is also an intermembrane space between the outer and inner membranes

Describe the enzymes of the respiratory complex

The respiratory chain, also known as the Electron Transport Chain (ETC), comprises four multi-subunit enzyme complexes embedded in the inner mitochondrial membrane

These are:

• Complex I: NADH-Q reductase (NADH dehydrogenase)

• Succinate-Q reductase (Succinate dehydrogenase)

• Complex III: QH2-cytochrome c reductase (Ubiquinone: cytochrome c oxidoreductase

• Complex IV: Cytochrome c oxidase These complexes contain various prosthetic groups that participate in electron transfer. Succinate dehydrogenase (Complex II) also functions in the citric acid cycle

Identify the protein complexes of the electron transport chain

Complex I: NADH-Q reductase

Complex II: Succinate-Q reductase

Complex III: QH2-cytochrome c reductase

Complex IV: Cytochrome c oxidase

Describe the features and components of the protein complexes

Complex I (NADH-Q reductase): A flavoprotein using FMN and contains non-heme iron centers (Fe-S complexes) . It transfers 2 electrons from NADH to coenzyme Q and pumps 4 H+ out of the matrix per 2 electrons transferred.

Complex II (Succinate-Q reductase): Couples the oxidation of succinate to fumarate (in the citric acid cycle) with the reduction of ubiquinone. It contains FAD, heme molecules, Fe-S centers, and cyt b. It does not act as a proton pump

Complex III (QH2-cytochrome c reductase): Couples the transfer of 2 electrons from ubiquinol to cytochrome c. It contains cyt b, cyt c1, and an Fe-S center. It acts as a proton pump, moving 4 H+ out of the matrix per molecule of QH2 converted to coenzyme Q.Electrons are passed one at a time to two molecules of cyt c

Complex IV (Cytochrome c oxidase): Carries electrons from cyt c to molecular oxygen, reducing it to H2O. It contains cyt a, cyt a3, and two Cu ions (CuA, CuB)6. For every molecule of O2 reduced, 4 H+ are ejected into the intermembrane space

Explain the chemiosmotic theory of proton movement

The chemiosmotic theory states that transmembrane differences in proton concentration are the reservoir for the energy extracted from biological oxidation reactions. As electrons flow through the respiratory complexes (I, III, and IV), protons are pumped from the mitochondrial matrix into the inter-membrane space, creating an electrochemical proton gradient. This gradient, also known as the proton motive force, drives the protons back into the matrix through ATP synthase, which catalyzes the synthesis of ATP. Oxidation and phosphorylation are coupled through this gradient

Describe proton motive force

• Proton motive force is the electrochemical proton gradient across the mitochondrial inner membrane.

• It is generated as protons are pumped out of the matrix into the inter-membrane space during electron transport.

• It has two components: a concentration gradient (ConcH+ outside >> ConcH+ inside) and a potential gradient (more positive charge outside).

• This force drives the H+ back into the matrix through ATP synthase, releasing energy for ATP synthesis

Describe the role of ATP synthase

• ATP synthase (Complex V) catalyzes the synthesis of ATP from ADP and Pi as protons flow passively back into the mitochondrial matrix through its proton pore

• It has two main domains: F0 (spanning the inner membrane, forming a proton channel) and F1 (located on the matrix side, where ATP synthesis occurs)

• The flow of H+ through F0 causes a rotation that drives conformational changes in F1, leading to the binding of ADP and Pi and their conversion to ATP

Describe the production of ATP from NADH, FADH2

• NADH and FADH2 donate electrons to the electron transport chain. As electrons are passed through Complexes I (for NADH), III, and IV

• protons are pumped into the inter-membrane space, generating a proton motive force.

• Electrons from FADH2 enter the chain at Complex II, which does not pump protons

• The flow of protons back into the matrix through ATP synthase drives ATP synthesis .

• The source provides approximate ATP yields: 2.5 ATP per NADH (rounded to 3 ATP) and 1.5 ATP per FAD (rounded to 2 ATP), based on the number of protons pumped and required by ATP synthase